Cascaded H bridge based solid state transformer for Medium Voltage and Low voltage interface, Simulation and Analysis.

This thesis paper presents a smart way to interface a medium voltage grid with low voltage grid as a response to rapidly growing penetration of distributed Energy resources (DES) in the power system grid. With renewable energy resources and distributed storages ever increasing, the complexity to manage loads connected to either power grid or the distributed resources have to be addressed in efficient manner and thus the concept of Solid-State Transformer (SST). Added functionalities such as bidirectional power flow between the grids, end voltage regulation, active and reactive power compensation, source side voltage sag compensation, etc. have made the use of SST even more attractive. Often called power electronics transformer, SST is a three-stage device that consists of power semiconductor converter and a high frequency transformer used for interconnection between grids and/or grid and the load. Numerous topologies to structure the SST configurations have been published in several papers. A concise literature review of available topologies along with power semiconductor devices that can be used for development of SST has been carried out in this paper as well. The primary objective of this thesis has been to design an SST with multilevel converters that can interface medium and low voltage level with feedback control on the medium voltage side of the device. The designed SST is simulated in MATLAB/SIMULINK to verify its operation. The designed is made to interface 7.2KV medium voltage level with 400V low voltage level. The design includes AC/DC multilevel front-end rectifier, isolated DC/DC converter with bidirectional power flow and DC/AC inverter.